Digital printing apparatus using liquid toner dispersion—also known as liquid toner—are known from U.S. patent application publication no. 2011/0249990. The known digital printing apparatus comprises a feed member, a development member, development member cleaning means, and an image carrying member, these member being preferably rollers; the feed member being arranged to transfer a quantity of liquid toner from a toner container onto the toner member; and the development member being arranged to transfer a portion of the quantity of liquid toner onto the image carrying member in accordance with a charge pattern sustained on a surface of said image carrying member. A liquid toner residue, also referred to as an excess liquid toner, remains on (the surface of) the development member after the imagewise transfer of the liquid toner from the development member to a further member, particularly the imaging member.
After transfer to the imaging member, the liquid toner dispersion is transferred to the substrate, either directly or via a further member. This transfer process is carried out for each colour separately. The liquid toner dispersion is subsequently fused in a fusing unit, resulting in a film according to a desired pattern, and hence an image on the substrate. Fusing may be carried out in various manners and typically involves heating.
Fusing of the liquid toner is one of the sensitive steps in the digital printing process. This fusing is to result in coalescence of the toner particles on the substrate. The term ‘coalescence’ refers herein to the process wherein toner particles melt together and form a film or continuous phase that adheres well to the substrate and that is separated from any carrier liquid. The fusing has to avoid formation of an emulsion, since an emulsion does not give a good printing image.
A particular issue resulting in misprints, is known as ghost fusing images. These are images that are different in colours and/or portions of the image being slightly shifted in gloss. It is necessary for high quality printing to prevent such ghost images, without reducing of printing speed of the digital printing apparatus, i.e. a high-speed of the substrate moving through the printing apparatus, for instance of at least 50 cm/s, preferably at least 70 cm/s or even at least 1 m/s.